Watching The Trades

If you ask an engineer a pipe is vessel and a tube is a structure. The standard measure for a pipe (i.e, the size you use to spec it) is the inside diameter because it is a vessel for transporting gas or liquids. The standard measure for a tube is measured by outside diameter because it is used for structural support.

If you ask a plumber, a pipe is any thick walled tube that can be threaded. A tube is anything that carries gas or liquids and is too thin walled to be threaded. Most plastic pipe is not threaded. However it is thick walled and that is why it is called pipe (You can also get plastic tubing).

In general, a plumber use inside diameter as the standard measures for both pipes and tubing. But Black Iron (and other ferrous metal pipes) and plastic pipes nominal size is only an approximation of the inside diameter. What’s more, the inside diameter will vary by thickness of the pipe wall (which is indicated by the pipe schedule). So a schedule 40 pipe will have an inside diameter that is almost twice as big as a schedule as a schedule 120 pipe even though they are both the same nominal size. To make matters worse, from 14” the nominal size is based on outside diameter.

To make matters even more complicated, the standard measure for annealed copper refrigeration tubing (otherwise known as ACR tubing) is always outside diameter. But this stuff is used by HVAC technicians, not your average plumber.

I don’t understand I why we all can’t be more like engineers. Do we really need a bunch of different standard measures for pipes when they are all used for transporting gas or liquids?

The answer that I have in a book called Plumbing a House is four wraps. I have seen this rule applied in real life by real people and their work has not leaked. So all is cool right?

Well, the issue is more complicated than that. Some people argue that for the bigger diameter pipes you need to wrap them more times around. If this is true, how many wraps should correspond to what diameter of threaded pipe?

While doing research on this very question I came across information calling into question the four wrap rule. In fact, the most common recommendation on the web seems to be 2 wraps. This comment from a forum is typical……

Many years ago a study was made on a large irrigation system that had numerous leaks. It was found the more leaks were caused by excessive teflon tape, than anything else. Two wraps was deemed adequate. Any more and the fittings were stressed.

Now I was not able to find that study on the net so I have no idea of how accurately this guy is reporting the findings. Nor do I know how rigorous the study was. But many people on the net seem to share his opinion that anything more then two wraps is excessive.

I even came across people who seem to cite the same study that he cited. But nobody seems to want to give the study its proper name. They always just refer to it as a study of an irrigation system. How aggravating.

One of the reasons that I want to see the actual study is that it is my understanding that a lot of agricultural irrigation systems use threaded plastic pipe. It seems to me that the rules for threaded plastic pipe and threaded steel pipe might be considerably different. I can easily see how four wraps of Teflon tape might stress a plastic fitting. I am little more iffy on the concept that four wraps of Teflon is going to stress a 2″ steel fitting designed to handle high pressure steam.

So, should you vary the amount of Teflon you use by the type of joint you are working on? I know one tradesman who argues that you should. Well, maybe argues is too strong of word. He just says that it is something that he does just “to be safe.” He uses the four wrap rule on the smaller diameters but on the bigger diameters he use up to 8 wraps (depending on the size).

But I know better than to go by someone’s “just to be safe” rule. A lot of tradesmen don’t really understand how the materials they are using work, so they just default to the rule that more is better.

Sometimes this rule of thumb works all right. If you are not sure how big your braces should be, err on the side of making them too big. But sometimes more of something can be harmful. So if you don’t know how or why your material works, you don’t really know if more is helpful or harmful.

The problem is that there is some dispute over how and why Teflon tape works. (more…)